In a therapy method based on a particle beam, a high-energy particle beam such as a proton beam or a carbon beam, which is accelerated up to 70% of the light speed, is utilized. These high-energy particle beams have the following features when irradiated into a body. Firstly, almost all of irradiated particles stop at a position of the depth proportional to the 1.7th power of the charged particle energy. Secondly, the density (referred to as a dose) of energy, which is given to the path through which an irradiated particle passes until it stops in a body, becomes maximum at a position where the particle beam stops. A distinctive deep dose distribution curve formed along a path through which a particle beam passes is referred to as a Bragg curve and the position where the dose value becomes maximum is referred to as a Bragg peak.
In a particle beam irradiation system utilizing the irradiation field forming method, which is frequently used and referred as the two-dimensional irradiation method, first, a particle beam is spread laterally so as to form a uniform lateral irradiation field, then, by using a patient collimator or a multi-leaf collimator, the irradiation field is formed in accordance with a shape of an affected part. Further, in order to conform the maximum depth position which is a position where a particle beam stops in a body (a position of Bragg peak) to the vicinity of an edge of an affected part regardless of a lateral position, a particle beam is made to pass through an energy compensation filter which is made for each patient (referred as a patient bolus or Bolus). Further, by using an apparatus called a ridge filter, the width of Bragg peak is spread so as to cover whole of the patient's depth width. By performing the above-mentioned, in a patient's volume, substantially uniform dose distribution can be formed.
A method for spread of a particle beam in a lateral direction includes a method using a scattering member, a method using a wobbler electromagnet, etc. A wobbler method using a wobbler electromagnet includes a single circular wobbler method in which a beam spot having approximately 10 centimeters is rotated along a circular path at about 50 Hz so as to form a uniform dose distribution in the center and a method for forming a uniform dose distribution in the center by scanning a beam spot having the diameter approximately 0.5 to 2 centimeters at a high speed in accordance with a complicated scanning pattern. According to this method, a thin beam spot is scanned with a fixed periodic pattern so as to form a uniform lateral dose distribution. Therefore, this method is also referred as a uniform scanning. As a scanning pattern, a spiral pattern and a saw wave pattern are well known. In a case where a scanning pattern is spiral, a method is called as a spiral wobbler method.